This invention relates to frequency error detection of detecting, in quadrature demodulation of a received signal derived by reception of a transmitted signal subjected to digital angle modulation of a transmission carrier by a symbol sequence to have a constant phase difference during each symbol period, a frequency error which a locally generated frequency has relative to a reception frequency of a reception carrier carrying the received signal. Such quadrature demodulation is known in the art as described, for example, in Japanese Patent Prepublication (A) No. 23,742 of 1991.
Portable telephone and like mobile communication systems are recently in wide use. In a radio communication network like the mobile communication system, communication between a transmitter station and mobile stations is bidirectionally carried out by transmitting from the transmitter station a transmitted signal on a transmission carrier of a predetermined carrier frequency.
For transmission of the transmitted signal to the mobile stations, the transmission carrier is digital angle modulated by a predetermined symbol sequence of a symbol rate to provide a preamble in the transmitted signal. Each mobile station receives the transmitted signal as a received signal with the transmission carrier received as a reception carrier for quadrature demodulation of the received signal. For use in the quadrature demodulation, each mobile station comprises a local oscillator for generating a local signal of a local frequency related to the predetermined carrier frequency.
The transmission carrier has a transmission frequency which may be subjected to a slight shift from the predetermined carrier frequency. The reception carrier therefore has a reception frequency which may be a little different from the predetermined carrier frequency. Furthermore, the local frequency is liable to fluctuations from the predetermined carrier frequency.
The frequency error must consequently be detected for the quadrature demodulation. It is possible on detecting the frequency error to use the fact that the transmitted signal has a constant phase difference in each symbol period while the transmission carrier is modulated by the predetermined symbol sequence and that the received signal has the constant phase difference meanwhile. Incidentally, the digital angle modulation may be GMSK (Gaussian filter minimum shift keying) modulation, MSK (minimum (phase) shift keying) modulation, or like modulation.
In the manner which will later be described in greater detail, a conventional frequency error detecting method comprises the steps of: (a) sampling the received signal by a sampling signal of a sampling period into a predetermined number of sampled vectors in each burst, (b) calculating phase errors, each phase error accompanying a phase difference between two adjacent ones of the sampled vectors, (c) calculating an average of the phase errors as a dividend, and (d) dividing the dividend by a divisor comprising the sampling period. It is possible to understand the predetermined number as 2m without loss of generality and to refer to the phase errors as zeroth through (2m-2)-th phase errors.
The average theoretically depends only on a total phase error accompanying a total phase difference between the first and the 2m-th sampled vectors with no regard to the second through the (2m-1)-th sampled vectors. Each sampled vector may, however, be influenced by a noise component. If the first or the 2m-th sampled vector is influenced by an appreciable noise component, the frequency error is influenced by the noise components and has a detection precision liable to fluctuations resulting from the noise components accompanying the received signal.